The present invention relates to a thermal fuse attached to prevent electronic equipment and electric appliances for home use from attaining to an abnormally high temperature.
Structure and function of a thermal fuse will be described with reference to FIGS. 1 and 2. FIG. 1 is a cross section of the thermal fuse in a normal state, and FIG. 2 is a cross section after operation. As shown in FIG. 1, the thermal fuse includes, as main components, a metal case 1, leads 2 and 3, an insulating member 5, compression springs 8 and 9, a movable electrode 4 and a thermosensitive material 7. Movable electrode 4 is movable while in contact with an inner surface of metal case 1 which is conductive. Between movable electrode 4 and insulating member 5, compression spring 8 is provided, and between movable electrode 4 and thermosensitive material 7, compression spring 9 is provided. In a normal state, compression springs 8 and 9 are each in compressed states. As compression spring 8 is stronger than compression spring 9, movable electrode 4 is biased to the side of insulating member 5, and movable electrode 4 is in pressure contact with lead 2. Therefore, when leads 2 and 3 are connected to an electric wiring of electronic equipment, for example, a current flows from lead 2 to movable electrode 4, from movable electrode 4 to metal case 1, and from metal case 1 to lead 3, thus conducting power. As the thermosensitive material, an organic substance, for example, adipic acid having a melting point of 150xc2x0 C. may be used. When a prescribed operating temperature is attained, thermosensitive material 7 softens or melts, and deforms because of the load from compression spring 9. Therefore, when electronic equipment or the like to which the thermal fuse is connected is overheated to reach the prescribed operation temperature, thermosensitive material 7 deforms and unloads compression spring 9. As compression spring 9 expands, compressed state of compression spring 8 is released in response, and as compression spring 8 expands, movable electrode 4 is separated from lead 2, thus cutting current, as shown in FIG. 2. By connecting the thermal fuse having such a function to an electric wire of electronic equipment and the like, damage to the equipment body or fire caused by abnormal overheating of the equipment can be prevented.
When the temperature to which the thermal fuse is connected increases rapidly, thermosensitive material 7 quickly softens, melts and deforms, and therefore lead 2 and movable electrode 4 are quickly separated. When the temperature rises gradually, however, thermosensitive material 7 softens, melts and deforms gradually, and therefore separation between lead 2 and movable electrode 4 proceeds gradually as well. As a result, a slight arc tends to be generated locally between lead 2 and movable electrode 4, which arc possibly causes welding contact between movable electrode 4 and lead 2, causing a problem that the function of the thermal fuse is lost.
When Agxe2x80x94CdO is selected as the material of movable electrode 4, for example, Agxe2x80x94CdO is superior in that it has low electric resistance and high thermal conductivity. When an arc is generated between lead 2 and movable electrode 4, however, there arises a problem that the welding contact phenomenon with lead 2 tends to occur, as CdO is significantly volatilized and sublimated in a closed space by the arc as CdO has high vapor pressure and movable electrode 4 formed of Agxe2x80x94CdO is apt to be deformed.
Such a problem of welding contact may be improved by increasing content of CdO in Agxe2x80x94CdO. When the content of CdO is increased, however, contact resistance with lead 2 increases, so that temperature at the contact portion tends to be increased. Thus, performance of the thermal fuse degrades.
When an Ag alloy oxide material is used as the material of movable electrode 4, the problem of welding contact is less likely when the oxide dispersed in the Ag alloy oxide material is fine particles. The oxide as the fine particles, however, increases contact resistance with lead 2, and as the temperature at the contact portion increases, the above described problem of degraded performance of the thermal fuse results.
An object of the present invention is to provide a thermal fuse that is free of any trouble of welding contact between the movable electrode and lead 2, even when the temperature of the equipment to which the thermal fuse is connected rises gradually, and that has small electric resistance at the time of conduction.
The present invention provides a thermal fuse in which a thermosensitive material is melt at an operation temperature to unload a compression spring, and by the expansion of the compression spring, a movable electrode and a lead that have been in pressure contact by the compression spring are separated to stop electric current, characterized in that the material of the movable electrode is obtained by performing internal oxidation process of an alloy having a composition containing 99 to 80 parts by weight of Ag and 1 to 20 parts by weight of Cu, that thickness of a layer having smaller amount of oxide particles at a surface of the material is at most 5 xcexcm, and that average grain diameter of oxide particles in the material is 0.5 to 5 xcexcm.
Preferably, the internal oxidation process is performed at an oxygen partial pressure of 0.3 to 2 MPa.
In the thermal fuse in accordance with the present invention, the material of the movable electrode may be an alloy having a composition containing 0.1 to 5 parts by weight of at least one of Sn and In.
The material of the movable electrode may be an alloy of a composition containing 0.01 to 1 parts by weight of at least one selected from the group consisting of Fe, Co, Ni and Ti.
In the present invention, the material of the movable electrode is preferably an alloy of a composition containing 0.1 to 5 parts by weight of at least one of Sn and In and 0.01 to 1 parts by weight of at least one selected from the group consisting of Fe, Co, Ni and Ti.